Concrete decking placed around swimming pools is generally on the order of about four inch thickness divided into six foot by six foot bays to prevent adverse, unsightly cracking. This is effected by introducing a wecond plain system comprising placement of a contraction joint which penetrates the concrete when wet through one-third of its thickness. The resulting benefit is that the concrete tends to be weaker in and about the straight lines representing the contraction joint.
It is recognized that concrete when poured begins to hydrate and become chemically hot, so that during the first few hours of setting, the decking is at its most expanded state. As cooling begins, the concrete begins to contract and the wecond plain lines allow for contraction. The shrinkage cracks remain invisible beneath the contraction joint. By also extending the joint around the cantilevered face of the decking, the contraction crack thereat occurs unnoticed behind the nose cap.
Prior to about 1958 when plastic contraction joints were first introduced to the trade, concrete workers utilized a deep jointer displaced across the top of the freshly poured concrete to form the contraction joint. This jointer was attached to a long handle and produced a joint 1 to 11/2 inches deep in straight lines across the concrete surface causing a wecond plain that allowed for concrete shrinkage. This method is still widely used throughout the concrete industry on, for example, sidewalks, wall caps, steps and anywhere controlled cracking of concrete is required.